While membrane separation is viewed as an energy and capital efficient process, present membrane technologies can often fall short of performance requirements for many separation and reaction tasks. For example, polymeric membranes can be degraded by hydrocarbons that exist in many industrial processes. The polymeric membranes operate at relative low temperatures. The permeation flux and selectivity of the polymeric membrane is relatively low. Inorganic and ceramic membranes, including zeolites, supported on porous substrates can provide high permeation flux and separation selectivity while exhibiting chemical and thermal stability. However, they are typically fragile and are generally made as membrane tubes with a surface area packing density much smaller than the polymeric membranes.
Zeolites are made of in-expensive raw materials and possess some unique properties, such as molecular sieving functions and stability. Particularly, the zeolite framework is stable in hydrocarbon or organic solvent medium and at elevated temperatures. These are very desirable attributes as a membrane material. Thus, there has been a strong interest to making zeolite membranes worldwide. The feasibility of making zeolite membranes and their unique molecular-sieving functions were demonstrated in late 1990s and early 2000s. The reported flux and selectivity of a zeolite membrane can be one or two orders of magnitude higher than the polymeric membranes for solvent dehydration. Mostly small ceramic disks and single-hole tubes were used as a support to elucidate membrane preparation and structures, and to demonstrate basic separation process concepts. Such supporting materials served those research purposes well. For actual membrane product development, however, novel product designs and manufacturing processes are needed to produce large membrane areas suitable for industrial applications at a cost competitive with existing separation technologies.
For recent two decades, there has been research publications around NaA-type and other zeolite membranes. The NaA membrane supported on ceramic alumina tubes has been commercialized for ethanol and solvent dehydration. Instead of single holes, several holes can be made into one membrane tube to increase the membrane area packing density. However, the tubular membrane cost cited in the literature is viewed too high for widespread usage. Membrane area packing density is another key issue to applications requiring large membrane areas.
The area packing density increases with decreasing the tube diameter. Exploratory studies of capillary inorganic membrane tubes have been reported with an attempt to achieve dramatic enhancement of the membrane area packing density. The feasibility to deposit a quality NaA membrane on an alumina hollow fiber of 1.2 mm O.D×0.6 mm thickness was shown. The ceramic capillary tubes tend to be brittle. An alternative is development of ceramic monolithic membranes. In the monolithic membrane body, a number of small membrane channels (<1.0 mm) are embedded in a sturdy, porous ceramic matrix so that making and packaging of individual, fragile capillary tubes is avoided, and manufacturing productivity of the membrane can be enhanced at the same time. The monolithic designs represent promising progress toward getting the surface area packing density of inorganic membranes close to polymeric hollow fiber membranes.
The attempt to make flat sheet zeolite membranes has been reported in the literature using sintered porous metal plates and metal meshes as a support. However, those porous metal supporting structures had rough pores and were too thick. The thick support is associated with high metal material costs and mass transport resistance. The rough pore of the support requires thick coating of modification and/or membrane layer. The thick coating adds membrane preparation complexity and presents potential adhesion/crack problems.
Inorganic membranes can have distinct advantages regarding resistance to degradation by various chemicals, stability at elevated temperatures, and high permeation flux and selectivity. Inorganic membranes are typically tubular in shape or are thick coatings on thick substrates. Tubes are commonly associated with relatively lower surface area packing density and higher cost per unit membrane area and engineering cost. Thick membranes have traditionally been important to seal defects such as pinholes and void structures present in thin membrane films, which are typically caused by less-than-ideal preparation procedures for the substrate structure and membrane. The thicker membranes generally provide low permeation flux and are associated with adhesion problems when the membrane and substrate are two different kinds of materials. For example, thermal mismatch between the membrane coating and substrate material can become pronounced with increasing the membrane thickness. A thick substrate (1 mm or above) is typically used in the conventional zeolite membrane synthesis due to the strength requirement. Porous ceramic tubes or disks are fragile and can easily be broken if made thin. Conventional metal screens or foams have large pores and are weak if made thin. Furthermore, thick substrates can increase the cost and weight of the membrane structure. Thicker substrates also impose additional resistance for the permeate to move through. These issues, and others, have been a major barrier hindering the development of efficient, thin, inorganic membranes having widespread applicability.